PR and QS are perpendicular to each other because PQ and QR are equal
sides to the triangle PQR and PS = SR, so QS bisects line PR evenly.
That means the angle PSQ is 70 and the angle RPS is 20. Now consider
the triangle PQS alone. If you know that QP and QS are equal, then
you know that angles QPS and QSP have to be equal. If QSP is 70,
then 20 + x = 70, and x = 50 (C).